The nominal pressure is the nominal pressure to which the tire is inflated and as defined, for example, by the Tire and Rim Association or TRA, standard.
The nominal deflection of a tire is, by definition, its radial deformation, or variation in radial height, as it changes from an unladened inflated state to a statically loaded inflated state under the nominal pressure and load conditions defined, for example, in the TRA standard. It is expressed in the form of a relative deflection, defined as the ratio of this variation in radial height of the tire to half the difference between the outside diameter of the tire and the maximum diameter of the rim measured on the rim flange. The outside diameter of the tire is measured under static conditions in an unladened state inflated to the nominal pressure. The TRA standard in particular defines the flattening of an aeroplane tire in terms of its flattened radius, which means the distance between the axis of the wheel of the tire and the plane of the ground with which the tire is in contact under nominal pressure and load conditions.
The following definitions hold true in what follows:    “Meridian plane”: a plane containing the axis of rotation of the tire.    “Equatorial plane”: the plane passing through the middle of the tread surface of the tire and perpendicular to the axis of rotation of the tire.    “Radial direction”: a direction perpendicular to the axis of rotation of the tire.    “Axial direction”: a direction parallel to the axis of rotation of the tire.    “Circumferential direction”: a direction perpendicular to a meridian plane and tangential to the tread surface of the tire.    “Radial distance”: a distance measured perpendicular to the axis of rotation of the tire and from the axis of rotation of the tire.    “Axial distance”: a distance measured parallel to the axis of rotation of the tire and from the equatorial plane.    “Radially”: in a radial direction.    “Axially”: in an axial direction.    “Radially on the inside of/radially on the outside of”: the radial distance of which is less/greater than.    “Axially on the inside/axially on the outside of”: which is at a lesser/greater axial distance.
A tire comprises a tread connected, by two sidewalls, to two beads intended to come in contact with a rim comprising two rim flanges. Each rim flange comprises a rim flange circular portion which is radially outermost, connected radially on the inside to a rim flange flat face the normal to which is directed substantially axially.
A radial tire more particularly comprises a reinforcement, comprising a crown reinforcement, radially on the inside of the tread, and a radial carcass reinforcement, radially on the inside of the crown reinforcement.
The radial carcass reinforcement of an aeroplane tire comprises a plurality of carcass reinforcement layers of which the axially outermost one in the bead is the carcass reinforcement external layer.
Each carcass reinforcement layer, made up of mutually parallel reinforcing elements which make an angle of between 80° and 100° with the circumferential direction, is wrapped, in each bead, around a bead wire core comprising a circumferential reinforcing element usually made of metal surrounded by at least one material which, non-exhaustively, may be polymer or textile. The meridian section of the bead wire core, which means the cross section through the bead wire core on a meridian plane, is inscribed inside a circle the centre of which is known as the centre of the bead wire core.
The carcass reinforcement layers, as described for example in document EP 1 381 525, usually comprise at least one layer known as the internal layer, which is wrapped around the bead wire core from the inside of the tire outwards to form a turned-back portion terminating in an end, and at least one layer known as the external layer, wrapped around the bead wire core from the outside of the tire inwards and axially on the outside, within the sidewall, of all the internal layers and of their respective turned-back portions.
The reinforcing elements of the carcass reinforcement layers, for aeroplane tires, are usually cords made of spun textile filaments, preferably made of aliphatic polyamides and/or of aromatic polyamides.
The tensile mechanical properties of the textile reinforcing elements (modulus, elongation and force at break) are measured after prior conditioning. What is meant by “prior conditioning” is that the textile reinforcing elements are stored for at least 24 hours, before measurement, in a standard atmosphere in accordance with European standard DIN EN 20139 (at a temperature of 20±2° C.; hygrometry of 65±2%). The measurements are taken in the known way using a tensile testing machine by ZWICK GmbH & Co (Germany) of type 1435 or type 1445. The textile reinforcing elements experience tension over an initial length of 400 mm at a nominal rate of 200 mm/min. All of the results are averaged over 10 measurements.
Each bead comprises a filling element which extends the bead wire core radially outwards. The filling element in any meridian plane has a substantially triangular meridian section exhibiting a radially external end and is made up of at least one filling polymer material. The filling element may consist of a stack in the radial direction of at least two filling polymer materials in contact with one another along a contact surface which intersects any meridian plane along a meridian line. The filling element in particular separates the internal layer which is axially closest to the bead wire from the turned-back portions and the layers axially on the outside of the said internal layer.
After curing, a polymer material is mechanically characterized by tensile stress-strain characteristics that are determined by tensile testing. These tensile tests are carried out, on a test specimen, in accordance with a method known to those skilled in the art, for example in accordance with international standard ISO 37, and under normal temperature (23+ or −2° C.) and hygrometry (50+ or −5% relative humidity) conditions defined by international standard ISO 471. The tensile stress of a polymer compound, measured for a 10% elongation of the test specimen and expressed in megapascals (MPa) is called the 10% elongation elastic modulus.
In use, the mechanical stresses of running induce bending cycles in the beads of the tire which wrap over the rim flanges.
Each bead, under the combined action of the nominal pressure and of the load applied to the tire which can vary between 0 and twice the nominal load, thus to adopts the geometry of the rim flange via its face axially on the outside of the bead wire core, known as the bead external face, which thus comes into contact with the rim flange.
The region of bending over the rim is the part of the bead the external face of which is intended to come into at least partial contact with the rim flange circular portion when the load applied to the tire inflated to its nominal pressure varies from 0 to twice the nominal load.
The bending cycles generate, in the carcass reinforcement layer portions situated in the region of bending over the rim, variations in curvature which are combined with variations in elongation. These variations in elongation or strain, particularly in the axially outermost carcass reinforcement layers, may have negative minimum values corresponding to their being placed in compression, and this may lead to fatigue rupture of the carcass reinforcement layer reinforcing elements and therefore degradation of the tire. The carcass reinforcement layers are then said to rupture through compression fatigue because of compressive strain which is too high in terms of absolute value, compressive strain being, by convention, negative.
The risk of a carcass reinforcement layer rupturing through compressive fatigue is higher the more axially external the carcass reinforcement layer is, which means the further away it is from the neutral axis of the bead considered like a beam in bending. Therefore, minimizing the risk of compressive fatigue rupture of the axially outermost carcass reinforcement layer or carcass reinforcement external layer makes it possible to minimize the risk of compressive fatigue rupture of the carcass reinforcement layers which are axially on the inside of the carcass reinforcement external layer and axially on the outside of the neutral axis of the bead.
The person skilled in the art also knows that the carcass reinforcement layers made up of reinforcing elements particularly comprising aromatic polyamides have low compression strength and are particularly susceptible to compressive fatigue rupture.